1. Field of the Invention
The present invention relates generally to a magnetic memory element having a magnetic tunnel junctions (MTJ) and particularly to a magnetic memory element having an MTJ with perpendicular anisotropy.
2. Background
Magnetic random access memory (MRAM) is rapidly gaining popularity as its use in replacing conventional memory is showing promise. Magnetic tunnel junctions (MTJs), an essential part of the MRAM, used to store information, are made of various layers, at least some of which determine the magnetic characteristic of the MRAM. An exemplary MTJ uses spin transfer torque to effectuate a change in the direction of magnetization of one or more free layers in the MTJ. That is, writing a bit (“1” or “0” in digital logic) of information is achieved by using a spin polarized current flowing through the MTJ, instead of using a magnetic field, to change states or program/write/erase the MRAM.
In spin transfer torque (STT) MTJ designs, when electrons flow across the MTJ stack, which is commonly referred to as “MTJ”, in a direction that is perpendicular to the film plane or from the pinned (sometimes referred to as “reference” or “fixed”) layer to the free (sometimes referred to as “switching” or “storage”) layer, spin torque from electrons transmitted from the pinned layer to the free layer orientates the free layer magnetization in a direction that is parallel to that of the reference (or pinned) layer. When electrons flow from the free layer to the pinned layer, spin torque from electrons that are reflected from the pinned layer back into the free layer orientates the free layer magnetization to be anti-parallel relative to the magnetization of the pinned layer. Thus, controlling the electron (current) flow direction causes switching of the direction of magnetization of the free layer. Resistance across the MTJ stack changes between low and high states based on the magnetization of the free layer, i.e. parallel versus anti-parallel, relative to that of the pinned layer.
However, a problem consistently experienced and preventing advancement of the use of MTJs in STTMRAM designs is reducing the threshold voltage or current used to switch the free layer magnetization during write operations. Such current and threshold voltage requirements limit the use of spin transfer torque-based MTJ devices in practical applications.
MTJs with perpendicular anisotropy, such that the magnetic moments of the free layer and the fixed layer thereof are in perpendicular directions relative to the plane of the film, are more appealing than their in-plane anisotropy counterparts largely due to the density and thermal stability improvements realized by the former.
While attempts are made to reduce the switching current and voltage of the MTJ, such attempts have largely addressed in-plane MTJ designs and not perpendicular MTJs. One way to reduce the switching current and voltage is to ease the switching of the free layer of an MTJ. Existing perpendicular MTJ designs include a free layer whose magnetic orientation (also referred to as anisotropy) relative to a reference (“fixed”) layer, while perpendicular in direction, has high effective coercivity field (Hc). Effective Hc throughout the free layer is non-uniform. That is, effective perpendicular coercivity field (Hc) of the free layer changes relative to the position within the free layer such that the center of the free layer generally has a lower effective Hc than the outer edges of the free layer with effective Hc essentially increasing from the center of the free layer to its outer edges.
Lower effective Hc of the free layer would allow easier switching of the free layer and would lower the threshold voltage and current required to switch the magnetization of the free layer.
Thus, the need arises for decreasing the overall effective perpendicular anisotropic field of the free layer of magnetic memory element and thereby reducing the threshold voltage and current required to operate the same, with minimal impact on thermal stability of the free layer.